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I'm planning on magnetizing a wraithlord/wraithknight in an upcoming Eldar detachment. I have some 1/8" by 1/16" magnets, but the guide I'm looking at is saying to use 1/4" by 1/16" magnets.

If I take two of the magnets I have and put them together, is the strength the same as one 1/4" by 1/16" magnet?

I would recommend that you get the bigger magnets. Putting 2 magnets one on top of the other doesn't increase the strength of the magnet what so ever. Simple makes it physically deeper, which could potentially be a big problem. The larger the surface area of the magnet the better it will hold without sagging because there is less unsupported plastic/resin. We ran into this problem with my wife's nids when were started magnetizing the arms with the smallest recommended magnets, the heavier arms tend to try and sag.

 DFTR_Remorse wrote:
Putting 2 magnets one on top of the other doesn't increase the strength of the magnet what so ever.

I have no idea what the relative strength of 1/4" vs 2 x 1/8" magnets are... but stacking magnets DOES increase their strength up to a point. You can easily test it, just grab a few magnets, try and pull 2 apart, then stack 4 together and try and pull them apart, it will take more force.

As for the original question... it's really hard to say. The force to pull 2 magnets directly apart usually isn't the problem with undersized magnets, typically the problem is magnets rotating against each other (so you mount a sword horizontal and it flops to the ground, but remains attached). OR, it falls apart because the magnets "peel" apart.

The main benefit of larger magnets is that the larger surface area provides more resistance to rotation and peeling.

I always just go for the biggest magnet I can fit in a joint because removing a magnet that is too small is just a pain.

Yeah, 'rotating' is more of an issue than strength. I find I often have to put 'pegs' for the magnetized part to stay still when attached.

I wish they would make magnets with teeth on the surfaces so when you attached them, they would basically have a rough surface to interlock and prevent rotation. I haven't seen them as a product but I do something like that for larger vehicular mounts.

For my Tyranids, I used the 1/8" magnets and put a stack of 3 in each arm. So far, they hold just fine, with no rotational problems. If you need to add some teeth to the magnets, you could try some textured paint and then covering it with a thin layer of superglue. I'd test that out on a spare piece of sprue first though.

My issue with magnets larger than 1/8" is simply this. I have a 1/8" drill bit that makes it very easy to prep holes for 1/8" magnets and most sockets can take a 1/8" hole pretty easily. 3/16" and 1/4" holes require more prep time, green stuff, etc., and not all plastic pieces will take a larger hole. As a result, I use stacks of smaller magnets.

What size tyranids do you do with 1/8"? I find the larger of my Trygon arms are pretty easy to rotate even 1/4" magnets.

The primary factor (besides grade) for the strength of the magnet is the contact diameter, not it's depth. Clicking another magnet to the back of a magnet pair may give you a few percent strength, but nothing like a milimeter of diameter.

AllSeeingSkink wrote:
What size tyranids do you do with 1/8"? I find the larger of my Trygon arms are pretty easy to rotate even 1/4" magnets.

Tervigons and Tyrants so far.

Here are the magnets I use: http://www.kjmagnetics.com/proddetail.asp?prod=D201

Their website has a nice magnetic force calculator too, which indicates that stacks of magnets do significantly improve the pull.

 Stephanius wrote:
The primary factor (besides grade) for the strength of the magnet is the contact diameter, not it's depth. Clicking another magnet to the back of a magnet pair may give you a few percent strength, but nothing like a milimeter of diameter.

Agreed. This is much of what I meant in my original post. Not that stacking magnets doesn't give you a boost in magnetic strength but that it's so subtle that it may as well not be counted at all. However, after some research I have found that stacking of magnets in specific fashioned where you forcibly create multiple poles that both attract and and repel that the strength of said magnetic field is likely 4 times the original strength of the individual magnets as a whole. However, this process requires a large amount of magnet stacking as well as finding a way to keep the repelling magnets from separating due to their opposite poles being in contact. If that makes any sense. So, as you said I recommend large surface area magnets rather than deeper magnets to nullify the empty space that causes sagging.

If you get the 1/4" N52 strength magnets from K&J they are VERY strong.

I use those for all my heavy duty applications, and the only thing that has required something more was a thunderhawk stand.

I have a list of the magnets I used when magnetizing my first wraithknight here:
http://stephanius40k.blogspot.de/2013/06/wraithknight-magnetized-with-full.html
The measurements are metric, but with 6mm being approx 1/4", I'm sure you can work it out, especially considering it's just a guidline. ;-]